Structural members composed of aluminum or aluminum alloy have advantages that, for example, the weight is light and the deformation can be easily performed in order to obtain a predetermined shape, because processes can be easily applied. The structural members are used in a variety of fields, for example, for carburetors and throttle bodies as constitutive parts of automobiles.
Such a structural member is manufactured as follows. That is, for example, a mold is filled with a molten metal of aluminum or aluminum alloy, and then the molten metal is cooled and hardened to provide a cast product. In order to improve the corrosion resistance, the surface of the cast product is coated with a corrosion-resistant coating film.
Conventionally, such a corrosion-resistant coating film is provided by immersing the cast product in a chromate treatment liquid containing CrO3, i.e., hexavalent chromium, as described in Patent Document 1. For this reason, hexavalent chromium is also contained in the corrosion-resistant coating film. However, hexavalent chromium is a substance which imposes the load on the environment. Therefore, in recent years, investigations have been made in order to form a corrosion-resistant coating film containing no hexavalent chromium.
From the above viewpoint, the present applicant has proposed the following invention in Patent Document 2. Specifically, a Zn underlayer is formed on a structural member composed of aluminum or aluminum alloy.
Subsequently, a chromate coating film (corrosion-resistant coating film) that contains trivalent chromium, is formed on the Zn underlayer.
Patent Document 1: Japanese Patent Publication No. 60-35432;
Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-76041.